The plugin moves the orders a customer placed as a guest into the customer's account as soon as the account is activated. The customer sees them in the order history, and you don't have to reassign them by hand.
It's easy to configure the plugin. See pictures.
1. Install the plugin and open its configure page. Enable 'Enable plugin' and press 'Save'.
'Enable Logging (nopcommerce log)' writes every assigned order to the nopCommerce log, so you can see which order was moved to which customer.

2. Set the registration method. Open Configuration > Settings > Customer settings and set 'Registration method' to 'Email validation'. The customer has to confirm the email address, and the orders are assigned after that.
With any other registration method the plugin assigns nothing, and the configure page shows a warning.
If you don't want to use 'Email validation', enable 'Assign orders without email validation' on the configure page. Be careful: the email address is not verified then, so anyone who registers with someone else's email receives that person's guest orders, including the name, address and phone number.
3. Good to know.
Only orders of guests with exactly the same billing email are assigned. Orders of registered customers are never moved.
Orders are assigned at the moment the account is activated. There is no bulk action for customers who were already active before you installed the plugin.
nopCommerce doesn't raise the activation event when an administrator approves a customer, so nothing is assigned on a store with the 'Admin approval' registration method.
The settings apply to all stores, and the plugin doesn't filter orders by store. In a multi-store installation, guest orders from every store are assigned to the customer.